In mobile networks, geographic regions are divided into cells where each cell covers a particular area. Network coverage is provided within the cell by a base station (BTS) which is an antenna which transmits and receives radio broadcasts for the network. Each BTS is connected to a Base Station Controller (BSC), which in turn is connected to a Mobile services Switching Centre (MSC) or “exchange” as it is called in a fixed line telephony network. Mobile communication devices within the cell communicate with the network by transmitting signals to the base station which are then routed through the MSC associated with that base station and on to their destination. A MSC can be connected to multiple base stations.
Each MSC has a visitor location register (VLR). The VLR stores the subscriber record of all mobile devices which are currently located within the cells of the BTSs that the MSC serves. The subscriber record includes details of the subscriptions and services associated with the device such as MSISDN (telephone number), IMSI, telephony details, SMS details, GPRS details, location information, forwarding parameters, call barrings etc. The MSC/VLR is provided with this subscriber record from the subscriber's HLR (Home Location Register), which is associated with the network to which the device/subscriber is registered, when the device first enters a cell served by the MSC. The HLR stores the subscriber record of all subscribers to the network along with details of the MSC serving the subscriber at that time.
While a mobile communication device is switched on, it is constantly transmitting signals to the network. When the device moves into a cell served by a different MSC/VLR, that cell and MSC/VLR will receive a signal from the communication device for the first time At this stage the device is not registered with the MSC/VLR and so the MSC/VLR does not hold a subscriber record for that mobile device. This results in the MSC/VLR not recognizing the mobile device. The signals from the mobile device include data which identify the subscriber and provide information about the subscriber's home network and its home location register (HLR). The MSC/VLR, on detecting the new mobile device in one of its cells, attempts to register the subscriber. Registration is implemented by the new MSC/VLR by extracting the subscriber and HLR identification data from the signal from the mobile device. The MSC/VLR contacts the subscriber's HLR to indicate that the subscriber has entered one of its cells and to request the subscriber's subscriber profile. In response, the HLR records the details of the new MSC/VLR in which the subscriber is located and uploads the subscriber's subscriber profile to the new MSC/VLR. The HLR also informs the MSC/VLR in which the device was previously located that the device has left is cells and that the subscriber's subscriber profile should be deleted. Thus the subscriber profile of the subscriber is only held by a single VLR (the VLR associated with the cell in which the subscriber is currently located). The process of registering with a new MSC is known as inter MSC handover.
Systems have been proposed in which at least part of the subscriber profile is retained on the old MSC/VLR in an inactive state for a predefined time period after the MSC has received an instruction from the HLR to delete the subscriber profile In such systems, if a subscriber returns to a MSC which it has recently left and its profile is retained in inactive form, the HLR only needs to transmit an instruction to reactivate the subscriber profile rather than transmit the entire subscriber profile. Such systems aim to reduce the regularity at which entire subscriber profiles are transmitted between MSCs and HLRs in order to reduce the consumption of bandwidth between these network components.
When a user attempts to make, a call to a called (target) subscriber, the call first arrives at the Gateway MSC (GMSC) of the called subscriber's home network, which takes on responsibility for routing the call to the current location of the subscriber. The GMSC interrogates the HLR (also in the subscriber's home network) for the location of the target subscriber. This is to identify the location of the target subscriber/device but also to identify a routing path that it must use to connect to the MSC that is serving the target subscriber/device. The HLR returns these details to the GMSC along with a Mobile Station Roaming Number (MSRN) which has been allocated to the target subscriber by the serving MSC.
While the GMSC is retrieving the routing information, the call is held at the GMSC. The GMSC then uses this routing information to establish a communication link between itself and the MSC serving the target subscriber. The MSRN of the target subscriber is provided to the MSC in order that it can connect the incoming call to the target subscriber. It can take several seconds to set up a communication link and this time period is extended when the subscriber is roaming in places geographically far away from the home network, for example in different countries, mainly due to the distance involved and the number switching centres through which the call must be passed.
It is possible that the target subscriber may have moved to a different cell provided by a base station which is served by a different MSC/VLR during the time taken to complete the call set-up procedure from the GMSC to the MSC/VLR. In such cases the call is routed to the MSC/VLR which the called party has just left since this is the routing information that was provided to the GMSC by the HLR. This situation occurs when the target subscriber registers with a new MSC/VLR after the HLR has transmitted the details of the previous MSC/VLR and roaming number to the GMSC. In this case, the GMSC will have been advised of the previous MSC/VLR which served the target subscriber/device and will attempt to connect the call to the previous MSC.
Typically, the registration procedure is faster than the call set up procedure and this is particularly the case when the set up period is extended due to large physical distances between the GMSC and the serving MSC/VLR. Thus, by the time the GMSC has set up a communication link with the MSC/VLR, the device has registered with a new MSC/VLR and the subscriber details and MSRN will have been deleted from the old MSC/VLR at which the call has arrived. Without the MSRN and the subscriber details, the old MSC/VLR is unable to identify and to contact the subscriber or apply any call forwardings and so the call is ended. This is known as a dropped call.
The call will also be dropped in those systems in which the subscriber profile is retained in an inactive state since the inactive data will not be identified when the VLR scans its database for subscribers currently communicating through its BTSs using the received MSRN.
It is noted that it can be problematic when devices move between MSC/VLRs at the time when a call is being set up to that device. In particular, this can result in call dropping and customers not being connected to their desired contacts. This is a problem for mobile network operators.